Semiconductor flow sensors are commonly used to measure the flow characteristics of a fluid or gas sample. In the field of gas chromatography, for example, semiconductor flow sensors are used to detect individual components of a fluid sample as it flows through a tube or capillary. The flow sensor can detect changes in a physical or chemical property of the fluid, such as the fluid's velocity or thermal conductivity, and produce an electrical signal representative of the property changes.
One such flow sensor utilizes an electrically resistive metal film as a sensing element. The metal film extends across a substrate channel provided for receiving a fluid flow from a manifold so that the film crosses through the fluid path and is entirely bathed in the fluid stream. To operate the sensor, the metal film is heated by passing an electric current through the film. Heat from the metal film is conducted to the moving fluid at a rate determined by the thermal conductance through the fluid to the substrate and to the manifold, and by the convective cooling of the moving fluid. For a given electrical input power, the temperature of the metal film depends in part on the properties of the fluid. Since the metal film will typically have a non-zero temperature coefficient of resistance, the temperature of the film can be monitored by measuring the electrical resistance of the film. Accordingly, transduction from fluid property to electrical resistance can be achieved.
Depending upon the application and the type of flow data desired, the sensor can be disposed so that the direction of fluid flow is either perpendicular to the plane in which the metal film lies, or parallel to that plane. Additional information can be obtained, such as flow velocity, by including a plurality of metal film elements disposed along the flow path of the fluid. By modulating the electric current provided to an upstream metal film element at a predetermined frequency, a thermal pattern corresponding to the frequency would be conducted into the fluid. Thus, the flow rate of the fluid can be determined by measuring the phase difference of the resistive changes detected at a downstream metal film element.
In order to obtain a high degree of sensor data reproducibility, the sensing element should be accurately positioned with respect to the fluid flow path. Moreover, the sensors should be sealed to withstand high operating temperatures and pressures. Accordingly, the attachment of the sensor to the manifold is critical to the proper functioning of the sensor. One conventional approach to providing a seal between the sensor and manifold is disclosed in U.S. Pat. No. 4,471,647 to Jerman et al., entitled GAS CHROMATOGRAPHY SYSTEM AND DETECTOR AND METHOD. Jerman teaches the use of a gasket which is clamped between the sensor and manifold.
While a gasket provides an adequate seal for certain applications, dimensional variations and inconsistencies of the gasket material can result in inaccurate positioning of the metal film with respect to the fluid flow. Precision alignment of the gasket with respect to the sensor and manifold during manufacturing is difficult to achieve, and is further exacerbated given the current trend toward miniaturization of gas chromatography devices. Misalignment of the sensor/manifold interface is known to cause undesirable perturbation of the fluid flow pattern with resulting degradation of measurement accuracy. Moreover, conventional gasket materials are often incapable of withstanding the increasingly high pressure (above 100 pounds per square inch) and temperature (above 450 degrees Celsius) demands placed upon current sensor capabilities.
Therefore, a critical need exists for a semiconductor flow sensor that is capable of forming a seal with the manifold. The flow sensor should provide for accurate alignment of the metal film to the fluid flow channel and should be chemically and mechanically stable at high temperature and pressure values. The sensor should also be relatively easy to batch fabricate at the semiconductor level.